1. Field of the Invention
The present invention relates to a press pad for use in a laminating press for the production of laminate sheets, such as decorative laminates, laminated floorboards, and punted circuit boards, using low pressure and high pressure single daylight and multi daylight presses
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
The purpose of a press pad is to compensate for density variations in the laminate sheet being pressed and thereby to ensure that an equal pressure is applied to all parts of the sheet. In addition, the press pad compensates for any unevenness in the surfaces of the platens of the press itself and any flexure or bow mg of the platens when under pressure. Again, this assists in the product on of a flat, even density laminate. Thus, it is important for a press pad to be resilient and have a natural springiness to permit it to compensate for the aforementioned density variations and the surface unevenness of the press platens but also to allow it to relax after each pressing operation and recover its form to enable it to be used again. The capacity a press pad has to re-form itself after each pressing, namely its compensation capability, is an important characteristic to ensure a reasonable working life and to avoid unnecessary downtime of a press whilst the press pads are replaced.
However, because the purpose of the press is to apply heat to the laminate sheet whilst it is under pressure, it is important that the press pad also conducts the heat supplied by the press platens to the laminate sheet. Working temperatures for such presses are usually in a range up to 220° C.
Typically, therefore, a conventional press pad is a densely woven combination of high temperature-resistant, non-asbestos yarns and metal wire. The metal wire is included to give good heat transmission through the pad to the laminate sheet. In contrast, the non-metal yarn is required to give the pad the springiness and resilience required to enable the pad to relax after each pressing operation. The relative proportion of the two types of material is a consideration when devising a press pad for a particular purpose. Usually a compromise must be reached between the heat transference and the resilience or springiness required in each case.
A conventional press pad is described in WO9613376. The pad comprises a woven fabric of heat resistant strands such as copper wires wherein a substantial proportion of either the warp or the weft comprises a silicone elastomer. In practice, as shown in FIG. 1, the warp 1 usually comprises stranded or bunched brass or copper wire and the weft 2 usually comprises a silicone covered metal wire, in particular a stranded or bunched wire core 3 over which a sheath 4 of silicone has been extruded. As a result of the presence of the silicone 4, this press pad has a great resilience and springiness whilst the metal wires ensure that the press pad achieves good heat transference from the platens to the material being pressed. One disadvantage of press pads of this type is that in use when pressurized in a laminating press the silicone sheath 4 tends to be cut through at the crossing points of the warp 1 and the well 2 by a scissor-action of the stranded or bunched wire core 3 and the metal wires of the warp 1. Eventually, this causes the pad to wear out because over time the weave structure is flattened to such an extent that the press pad is unable to relax after each pressing operation and the pad loses its resilience and springiness.
In WO2007129041 is described a press pad that mitigates the aforementioned problem. In this press pad the wires within the core 3 of the well are not stranded or bunched but are arranged to lie substantially parallel to one another and to the longitudinal axis of the core 3, as shown in FIG. 2. This enables the Wires of the core 3 to move relative to one another and therefore to flatten out, potentially into the same plane when the pad is pressurized when in use. This reduces the tendency of the warp wires 1 to cut into the silicone sheath 4 of the well 2.
Also described within WO2007129041 is the possibility of using a weft Wherein the wires 3 are arranged to lie in the same plane of the core when they are not under any applied pressure. This involves the extrusion of an elastomeric sheath 4 in a non-circular, cross-sectional profile, for example an oval cross-sectional profile. However, such a well is ribbon-like, being, significantly wider than it is thick. Weaving such wells to produce a satisfactory press pad is difficult as all wefts tend to twist during weaving so that the resulting press pad would have an uneven thickness and differently sized interstices between the warp and well owing to the twists in the weft. This is undesirable because the number of contact points between the warp and the well in any given surface area of the resulting pad is reduced which in turn reduces the compensation capabilities of the pad. Even if it Were possible to weave a pad with a ribbon-like well that remained untwisted, increasing the width of the well in any plain or twill weave results in a reduction in the number of well insertions or picks over a given length of woven fabric. This is also undesirable because the number of contact points between the warp and press platens in any given surface area of the resulting pad is thereby reduced which in turn reduces the beat transfer capabilities of the pad,
The primary object of the present invention is to provide a press pad wherein elastomeric material forms a greater percentage of the overall pad volume than in conventional pads and thereby produce an enhanced compensating ability or ‘spring’ effect without significantly reducing the heat transfer capability of the pad.